1. Field of the Invention
The present invention relates to high-strength steels having excellent fatigue characteristics, for valve springs, and a process for producing such high-strength steels.
2. Discussion of the Prior Art and Problems Solved by the Invention
Valve springs, generally in the form of a coil spring, used for an internal combustion engine of automotive vehicles are operated usually at temperatures in the neighborhood of 150.degree. C., and are subjected to compressive loads periodically applied at a high frequency. As such, the valve springs are considered one of springs that are used under the severest operating conditions.
A commonly known steel material for such valve springs is an oil-tempered wire such as SWO-V, SWOCV-V and SWOSC-V classified according to the Japanese Industrial Standards (JIS). Of these wires, the SWOSC-V wire (oil-tempered wire of silicon chromium steel for valve springs) is widely used as the material for valve springs suitable for internal combustion engines, since this wire exhibits higher fatigue strength and sag resistance (resistance to permanent set), than other oil-tempered wires used for valve springs for other applications. For further improvement in the fatigue strength, the wire is subjected to a nitriding or carbo-nitriding treatment to increase the surface hardness.
Recent developments of internal combustion engines are directed toward satisfying a need for higher output and speed of the engine. This tendency requires the valve springs to provide higher resistance to stresses and longer life expectancy, that is, improved reliability of the valve springs. To meet this requirement, it is desired to develop a steel material for valve springs, which is excellent in strength and fatigue characteristics. Attention is currently directed to the removal of inclusions in the steels, in an effort to improve their properties. For example, there has been an attempt to control the form of such inclusions, by means of ladle-furnace refining techniques such as ASEA-SKF process. Reports indicate that reduction in the size and quantity of the inclusions, and a change in the composition of the inclusions for increased ductility may be effective to improve the properties of a steel material. However, such an improved steel material having an increased strength still considerably suffers from a problem in terms of reliability, due to a large variation in the fatigue strength (.sigma.WB) in a relatively high hardness range (e.g., H.sub.B hardness higher than 400), and due to reduced fatigue life.